Method for maintaining the white colour point in a field-sequential LCD over time

• US 8,421,827 B2
• Filed: 03/05/2010
• Issued: 04/16/2013
• Est. Priority Date: 10/05/2004
• Status: Active Grant

• Current Assignee: Malikie Innovations Limited (Key Patent Innovations Limited)
• Original Assignee: Blackberry Limited
• Inventors: Drader, Marc, Robinson, James, Mailloux, Jerry, Lowles, Robert
• Primary Examiner(s): Lao, Lun-Yi
• Assistant Examiner(s): Hegarty, Kelly B
• Application Number:
  US12/718,626
• Publication Number:
  US 20100156962A1
• Time in Patent Office: 1,138 Days
• Field of Search: 345/690, 345/84, 345/87, 345/102, 345/204, 345/691
• US Class Current: 345/690
• CPC Class Codes: G09G 2320/0242 Compensation of deficiencies in the appearance of colours
• CPC Class Codes: G09G 2320/043 Preventing or counteracting the effects of ageing
• CPC Class Codes: G09G 2320/0666 for control of colour parameters, e.g. colour temperature
• CPC Class Codes: G09G 3/3413 Details of control of colour illumination sources

Abstract

A field sequential liquid crystal display maintains its white color point through compensation values to at least one color light emitting diode that illuminates the display. The compensation values may be impedances to control the current or pulsing of the current source according to a pulse width modulation technique. A degradation curve may be used to calculate extrapolate the theoretical forward voltage of the light emitting diode. Additional complexity arises from the need for calculating uptime for multiple light emitting diodes of different colors. Brightness levels may also be factored in. Additional processing of a display element may be provided when a grey scale image is being generated.

Claims

1. A method for maintaining a colour point for a plurality of light emitting elements used to illuminate a display of an electronic device, comprising:

comparing a first value corresponding to activation data of each element of the plurality of light emitting elements against a first threshold to identify a compensation element to compensate for aging of the light emitting elements, the activation data corresponding to one of the total time the plurality of light emitting elements have been activated and a function of activation time and an intensity value of the plurality of light emitting elements;

comparing the first value against a second threshold if the first value exceeds the first threshold;

if the first value is between the first and the second thresholds, then utilizing a first compensation value for the compensation value for aging the each element;

adjusting an output to produce the colour on the display by adjusting an intensity for the each element utilizing its compensation value; and

for a grey scale image to be generated on the display, at a pixel of the display setting the pixel to a transmissive state if the grey scale image at the pixel includes a colour to be activated.

2. The method of claim 1, further comprising:

if the first value exceeds the second threshold, then utilizing a second compensation value for the compensation value.

3. The method of claim 1, wherein the function includes a sum of a plurality of intensity products, wherein each product of the plurality of products is an activation time of the light emitting elements multiplied by intensities during the activation time.

4. The method of claim 1, wherein the compensation value relates to a first voltage drop across a first impedance element switched in series with the plurality of light emitting elements located in a circuit between power and ground.

5. The method of claim 4, wherein the compensation value is further related to one of:

a second voltage drop across a second impedance element switched in a parallel relationship with the plurality of light emitting elements;

a third voltage drop across a third impedance element switched in series with the plurality of light emitting elements located between power and ground; and

a fourth voltage drop across a fourth impedance element switched in a parallel relationship with the plurality of light emitting elements.

6. The method of claim 1, wherein adjusting the intensity for the each element utilizes a pulse width modulation signal derived from the compensation value.

7. The method of claim 1, wherein adjusting the intensity for the each element applies a voltage to one of:

elements in a line in the display;

a pixel in the display or a common electrode for a colour for the display.

8. The method of claim 7, wherein when the voltage is switched on the common electrode for the colour for the display, the voltage is switched for each colour of the display for each frame generated on the display.

9. The method of claim 7, wherein when the voltage is switched for elements in the line in the display, the line is alternatingly supplied through a source driver with voltages from a first set of a polarity and then supplied with voltages from a second set of a polarity opposite to that of the first set.

10. The method of claim 9, wherein when the voltage is switched for the pixel in the display, alternating columns for each row of the display are supplied with of opposing polarities.

11. The method for maintaining a colour point for a plurality of light emitting elements used to illuminate a display of an electronic device as claimed in claim 1, further comprising:

applying data and control signals to a column driver of the display to either set the pixel to the transmissive state or not turn on the pixel if the grey scale image at the pixel does not include the colour to be activated.

12. The method for maintaining a colour point for a plurality of light emitting elements used to illuminate a display of an electronic device as claimed in claim 1, further comprising:

switching a voltage applied to a common electrode for the display while the display is activated from a first bias voltage to a second, inverted bias voltage.

13. A field sequential liquid crystal display system that compensates for white colour point drift over time, comprising:

a liquid crystal display;

a light emitting element for illuminating the liquid crystal display, the white colour point drift of the liquid crystal display being compensated through compensation applied to the light emitting element;

a first module operating characteristics of the light emitting element to identify a compensation element to compensate for aging of the light emitting element by

comparing a first value corresponding to activation data the light emitting element against a first threshold, the activation data corresponding to one of;

the total time the light emitting element has been activated; and

a function of activation time and an intensity value of the light emitting element;

if the first value exceeds the first threshold, utilizing a first element for the compensation element;

compares the first value against a second threshold if the first value exceeds the first threshold; and

if the first value is between the first and the second thresholds, utilizing the first element for the compensation element;

a second module to adjust an intensity of an output of the light emitting element to compensate for the white colour point drift by adjusting an intensity of activation of the light emitting element by utilizing the compensation element; and

a third module to set a transmissivity state for a pixel in the display when the display is generating a colour selected from one of red, green and blue for a grey scale image, the state selected from one of;

a transmissive state if the grey scale image at the pixel includes the colour; and

a not turned on state at the pixel if the grey scale image at the pixel does not include the colour.

14. The field sequential liquid crystal display system of claim 13, wherein a voltage is switched on one of:

elements in a line in the display;

a pixel in the display or the common electrode for a colour for the display.

15. The field sequential liquid crystal display system of claim 14, wherein when the voltage is an inverted voltage signal applied to the elements in the line in the display, the line is supplied in through a source driver with voltages in an alternating manner from a first set of a polarity and then supplied with voltages from a second set of a polarity opposite to that of the first set.

16. The field sequential liquid crystal display system of claim 15, wherein when the voltage signal switched on the pixel in the display, alternating columns for each row of the display are supplied with voltage sets of opposing polarities.

17. The field sequential liquid crystal display system of claim 13, further comprising:

a fourth module to selectively switch a voltage applied to a common electrode for the display while the display is activated from a first bias voltage to a second, inverted bias voltage.

18. The field sequential liquid crystal display system of claim 13, wherein the compensation element is one of:

a first impedance element switched in a parallel relationship with the light emitting element;

a second impedance element switched in series with the light emitting element located between power and ground; and

a third impedance element switched in a parallel relationship with the light emitting element.

19. The field sequential liquid crystal display system of claim 13, wherein the first module further:

utilizes a second element for the compensation element if the first value exceeds the second threshold.

20. The field sequential liquid crystal display system of claim 19, wherein:

the first element is a first impedance element in a first switchable circuit in series with the light emitting element;

the second element is a second impedance element in a second switchable circuit in parallel with the light emitting element located between power and ground; and

the first and second switchable circuits are selectively connected to the circuit of the light emitting element to adjust the intensity of the output of the light emitting element to compensate for the white colour point drift.